A $1 billion desalination plant to supply San Diego County is under construction in Carlsbad and is scheduled to open as early as November.

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CARLSBAD, Calif. — Every time drought strikes California, residents cannot help noticing the substantial reservoir of untapped water lapping at their shores: 187 quintillion gallons of it, more or less, shimmering so invitingly in the sun.

Now, for the first time, a California metropolis is on the verge of turning the Pacific Ocean into an everyday source of drinking water. A $1 billion desalination plant to supply booming San Diego County is under construction in Carlsbad and is scheduled to open as early as November, providing a major test of whether California cities will be able to resort to the ocean to solve their water woes.

Across the Sun Belt, a technology once dismissed as too expensive and harmful to the environment is getting a second look. Texas, facing persistent dry conditions and a population influx, may build several ocean-desalination plants. Florida has one operating and may be forced to build others as a rising sea invades the state’s freshwater supplies.

In California, small desalination plants are up and running in a few towns. Plans are far along for a large plant in Huntington Beach that would supply water to populous Orange County. A mothballed plant in Santa Barbara may soon be reactivated. And more than a dozen communities along the coast are studying the issue.

The Carlsbad facility will be the largest ocean-desalination plant in the Western Hemisphere, producing about 50 million gallons of drinking water a day. So it is under scrutiny for whether it can operate without major problems.

“It was not an easy decision to build this plant,” said Mark Weston, chairman of the agency that supplies water to towns in San Diego County. “But it is turning out to be a spectacular choice. What we thought was on the expensive side 10 years ago is now affordable.”

Still, the plant illustrates many of the hard choices that states and communities face as they consider whether to tap the ocean for drinking water.

In San Diego County, which depends on imported freshwater supplies from the Colorado River and from Northern California, water bills average about $75 a month. The new plant will drive them up by $5 or so to secure a new supply equal to about 7 or 8 percent of the county’s water consumption.

The plant will use a huge amount of electricity, increasing the carbon-dioxide emissions that cause global warming, which further strains water supplies. And local environmental groups, which fought the plant, fear a substantial impact on sea life.

The company developing the plant, Poseidon Water, has promised to counter the environmental damage. For instance, it will pay into a California program that finances projects to offset emissions of greenhouse gases.

Still, some scientists and environmental groups contend that if rainy conditions return to California, the Carlsbad plant and others like it could become white elephants. Santa Barbara, northwest of Los Angeles, built its desalination plant 25 years ago and promptly shut it down when rains returned.

Australia is a more spectacular case: It built six huge desalination plants during a dry spell and has largely idled four of them though water customers remain saddled with several billion dollars’ worth of construction bills.

“Our position is that seawater desalination should be the option of last resort,” said Sean Bothwell, an attorney with the California Coastkeeper Alliance, an environmental coalition that has battled California’s turn toward the technology. “We need to fully use all the sustainable supplies that we have available to us first.”

The technological approach being employed in Carlsbad, and in most recent plants, is called reverse osmosis. It involves forcing seawater through a membrane with holes so tiny that the water molecules can pass through but larger salt molecules cannot.

A huge amount of energy is required to create enough pressure to shove the water through the membranes. But clever engineering has cut energy use of the plants in half in 20 years and improved their reliability.

Future desalination plants also have the potential to blend well with the rising percentage of renewable power on the electric grids in California and Texas. Since treated water can be stored, the plants could be dialed up at times when electricity from wind or solar power is plentiful, and later dialed down.

However, as interest in desalination spreads, California and other states confront major decisions about the environmental rules for the new plants.

The intake of seawater and the disposal of excess salt into the ocean can harm sea life. Sucking in huge amounts of seawater, for instance, can kill fish eggs and larvae by the billions. Technical solutions exist, but they can drive up costs, and it is still unclear how strict California regulators will be with the plant developers.

Environmental groups argue that the embrace of desalination represents a failure to manage freshwater effectively. They want much more aggressive programs focused on conservation and on reuse of existing supplies, pointing out that half of municipal water still goes to grass and other lawn plants. These arguments have sometimes carried the day, as they did when voters in Santa Cruz killed a desalination plant.

Long worried about water scarcity, the San Diego region helped to pioneer measures that ultimately spread across the country, including low-flow bathroom fixtures, more efficient washing machines and other innovations.

But these steps have not been enough to secure the region’s water future, Weston said. Thus the water authority decided years ago, long before the current drought began, to move forward on the desalination plant.

It is in the late stages of construction, by an artificial bay opening to the sea in Carlsbad. On a recent day, the faint odor of glue wafted through the air as workers sealed joints on huge pipes. When it goes into operation, the plant will pump water through 16,040 cylinders containing the membranes that trap salt.

Peter MacLaggan, a vice president from Poseidon Water who is overseeing the project, said the plant was in some ways a response to longstanding public interest in desalination.

“Every time California has a drought, we get letters to the editor pointing out that there’s a lot of water in the Pacific Ocean,” he said as waves broke on the shoreline in the distance. “They say, ‘Hey, guys, what are we waiting for?’ ”